Well, the core code for the fileno function looks like the following (which also handles the case of tied filehandles). It's basically PerlIO_fileno(IoIFP(GvIO((GV*)sv))) that you're after.

PP(pp_fileno)
{
    dSP; dTARGET;
    GV *gv;
    IO *io;
    PerlIO *fp;
    MAGIC  *mg;

    if (MAXARG < 1)
    RETPUSHUNDEF;
    gv = (GV*)POPs;

    if (gv && (io = GvIO(gv))
    && (mg = SvTIED_mg((SV*)io, PERL_MAGIC_tiedscalar)))
    {
    PUSHMARK(SP);
    XPUSHs(SvTIED_obj((SV*)io, mg));
    PUTBACK;
    ENTER;
    call_method("FILENO", G_SCALAR);
    LEAVE;
    SPAGAIN;
    RETURN;
    }

    if (!gv || !(io = GvIO(gv)) || !(fp = IoIFP(io))) {
    /* Can't do this because people seem to do things like
       defined(fileno($foo)) to check whether $foo is a valid fh.
      if (ckWARN2(WARN_UNOPENED,WARN_CLOSED))
          report_evil_fh(gv, io, PL_op->op_type);
        */
    RETPUSHUNDEF;
    }

    PUSHi(PerlIO_fileno(fp));
    RETURN;
}
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Dave.